This invention relates to battery chargers and more particularly, to an improved battery charger for a lead acid battery.
The ability of a lead-acid battery to deliver large amounts of electrical power is well known, particularly when associated with the starting of internal combustion engines. Likewise, the need to recharge these batteries and the problems associated therewith are also well known.
Many limitations and faults found in lead-acid batteries and other types of batteries are often the result of poor recharging control. Known battery chargers have various operating cycles to accommodate different battery charge states. For example, first, as a battery discharges, acid combines with the lead to form lead sulfate on the plates. This reduces the plate surface area and lowers the specific gravity of the electrolyte as well as the battery capacity. To properly charge the battery, all of the lead sulfate must be reacted or removed from the plates. Further, it is necessary that a battery charging cycle or process accommodate various degrees of lead sulfate plate deposits from readily removable lead sulfate deposits to lead sulfate deposits that are stubborn and more difficult to remove.
Second, to fully charge a battery, battery cells need to be charged above their nominal voltage in order to equalize their voltages. Such a battery charging cycle is referred to as an equalization charge cycle. Overcharging a battery to about 125% of its nominal voltage causes the electrolyte to bubble as hydrogen and oxygen are produced in the charging process. Such action also assists in the removal of sulfate particles from the plates as well as equalizing cell voltages. However, the charge current during an equalization cycle must be controlled. If the charge current is too great, overcharging occurs that wastes energy, causes excessive loss of electrolyte, often reduces the battery life, and may permanently damage the battery. On the other hand, undercharging the battery limits its capacity, thereby reducing its service life between charges and may lead to a degradation of the battery that is often unrecoverable.
Of particular interest is the use of wet cell or flooded lead acid batteries in marine applications, for example, a fishing boat. It is desirable that a battery be fully charged so that it lasts over a full day of fishing. Batteries with a shorter service life result in an unwanted interruption in the fishing activity; and such an interruption has significant consequences during a competitive fishing event. Currently, batteries on fishing boats are charged for several hours at night with first a standard battery charger. Thereafter, during the night, the battery is often charged with a second, high current charger to provide an equalization charge cycle. The use of a high current often stresses the battery with a severe overcharge condition.
There are known battery chargers that detect different levels of battery discharge and provide different battery charge cycles; however, such chargers utilize relatively complex circuits and are relatively expensive. Further, such battery chargers often do not charge a battery to its full potential charge without overcharging the battery. Consequently, there is a need for a simple and reliable charger for a lead acid battery that automatically and consistently provides a fully charged battery in response to a wide range of battery discharge conditions and ambient temperatures.
The present invention is an improved battery charger for lead acid batteries that consistently provides a fully charged battery independent of battery discharge condition. The battery charger of the present invention has the advantage of always providing the user with a battery that will have the longest possible service life. Further, the battery""s service life is not diminished by undercharging or overcharging. The battery charger of the present invention is particularly useful in marine applications, for example, on fishing boats. The battery charger of the present invention as the advantage of always providing a user with a battery that has the longest possible life between charges, thereby minimizing unwanted interruptions to the user""s activities and the consequences thereof.
According to the principles of the present invention and in accordance with the preferred embodiments, the invention provides a method of charging a lead acid battery by first applying a main charge current to the battery for a first time period that terminates upon a battery voltage rising to a first magnitude. Thereafter, applying an absorption charge current to the battery for a second time period determined as a function of, for example, one-half of, a time elapsing from an application of the main charge current until the battery voltage rises to the first magnitude. The length of time to charge a battery to a given voltage level is generally dependent on the degree of discharge of a battery. By automatically setting an absorption charge time proportional to the main charge time, the absorption charge time is automatically set to be generally dependent on the state of discharge of the battery. Therefore, this feature contributes to the capability of the battery charger to consistently provide fully charged batteries independent of the state of discharge.
In another embodiment, the invention provides a method of charging a lead acid battery by first applying a main charge current to the battery for a first time period that terminates upon a battery voltage rising to a first magnitude. Thereafter an absorption charge current is applied to the battery for a second time; and then an equalization charge current is applied to the battery for a third time period determined as a function of a length of the second time period. By automatically setting the equalization charge time generally proportional to the absorption charge time, the equalization charge time is automatically set to be generally dependent on the state of discharge of the battery. This feature also contributes to the capability of the charger to consistently provide fully charged batteries independent of the state of discharge.
In a further embodiment, the invention provides a method of charging a lead acid battery by detecting a first battery voltage before applying a charge current. After applying an evaluation current to the battery during an evaluation time period, a second battery voltage is detected. Next a deep discharge condition is determined to exist in response to the first battery voltage being below a first voltage reference and the second battery voltage being above a second voltage reference. A desulfation charge current is applied for a time period, for example, two hours, in response to determining a deep discharge condition. By automatically detecting a deep discharge condition and executing a desulfation charge cycle, the capability of the charger to consistently provide fully charged batteries is further enhanced.
In yet another embodiment, the invention provides a battery charger for a lead acid battery having a power supply with an input connected to an AC signal and an output connected to the battery. The power supply provides a charge current to the battery. A clock connected to the AC signal provides output clock pulses having transitions synchronized with zero crossings of the AC signal. A voltage monitor connected to the battery detects a battery voltage substantially simultaneously with a zero value of the charge current. A charge mode control is connected to the clock and the voltage monitor for commanding different battery charge currents. Measuring battery voltage in the absence of charge current provides a more accurate battery voltage measurement.
In one aspect of this embodiment, the voltage monitor has a temperature compensation circuit that uses a diode having a forward voltage drop with a temperature coefficient of about xe2x88x922 mv/xc2x0 C. at a constant current. A multiplier is connected to the diode and provides an output with a temperature coefficient of about xe2x88x924 mv/xc2x0 C. The voltage monitor has a voltage divider on one input that divides the battery voltage to that of one battery cell and a voltage reference input connected to the temperature compensation circuit. Therefore, the battery voltage measurement is temperature compensated to match the temperature coefficient of a flooded lead acid battery cell, that is, about xe2x88x924 mv/xc2x0 C. A more accurate battery voltage sampling better reflects the true battery voltage with the advantage of providing more consistently charged batteries under different environmental conditions.
In another aspect of the invention, the battery charger includes a module that can be placed at a location remote from the battery charger and convenient to the user. The module provides sensory perceptible indicators, for example, LEDs, representing operational states of the battery charger.
These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein.